Search space for non-interleaved r-pdcch

ABSTRACT

The present invention relates to providing control information within a search space for blind decoding in a multi-carrier communication system. In particular, the control information is carried within a sub-frame of the communication system, the sub-frame including a plurality of control channel elements. The control channel elements may be aggregated into candidates for blind decoding. The number of control channel elements in a candidate is called aggregation level. In accordance with the present invention, the candidates of lower aggregation levels are localised, meaning that the control channel elements of one candidate are located adjacently to each other in the frequency domain. Some candidates of the higher aggregation level(s) are distributed in the frequency.

FIELD OF THE INVENTION

The present invention relates to signalling uplink and downlink grant inan OFDM-based mobile communication system. In particular, the presentinvention relates to methods and apparatuses for configuration of searchspace and to search space channel structure for signalling of the uplinkand downlink grant control information.

BACKGROUND OF THE INVENTION

Third generation (3G) mobile systems, such as, for instance, universalmobile telecommunication systems (UMTS) standardized within the thirdgeneration partnership project (3GPP) have been based on wideband codedivision multiple access (WCDMA) radio access technology. Today, 3Gsystems are being deployed on a broad scale all around the world. Afterenhancing this technology by introducing high-speed downlink packetaccess (HSDPA) and an enhanced uplink, also referred to as high-speeduplink packet access (HSUPA), the next major step in evolution of theUMTS standard has brought the combination of orthogonal frequencydivision multiplexing (OFDM) for the downlink and single carrierfrequency division multiplexing access (SC-FDMA) for the uplink. Thissystem has been named long term evolution (LTE) since it has beenintended to cope with future technology evolutions.

The LTE system represents efficient packet based radio access and radioaccess networks that provide full IP-based functionalities with lowlatency and low cost. The detailed system requirements are given in 3GPPTR 25.913, “Requirements for evolved UTRA (E-UTRA) and evolved UTRAN(E-UTRAN),” v8.0.0, January 2009, (available at http://www.3gpp.org/ andincorporated herein by reference). The Downlink will support datamodulation schemes QPSK, 16QAM, and 64QAM and the Uplink will supportBPSK, QPSK, 8PSK and 16QAM.

LTE's network access is to be extremely flexible, using a number ofdefined channel bandwidths between 1.25 and 20 MHz, contrasted with UMTSterrestrial radio access (UTRA) fixed 5 MHz channels. Spectralefficiency is increased by up to four-fold compared with UTRA, andimprovements in architecture and signalling reduce round-trip latency.Multiple Input/Multiple Output (MIMO) antenna technology should enable10 times as many users per cell as 3GPP's original WCDMA radio accesstechnology. To suit as many frequency band allocation arrangements aspossible, both paired (frequency division duplex FDD) and unpaired (timedivision duplex TDD) band operation is supported. LTE can co-exist withearlier 3GPP radio technologies, even in adjacent channels, and callscan be handed over to and from all 3GPP's previous radio accesstechnologies.

FIG. 1 illustrates structure of a component carrier in LTE Release 8.The downlink component carrier of the 3GPP LTE Release 8 is sub-dividedin the time-frequency domain in so-called sub-frames 100 each of whichis divided into two downlink slots 110 and 120 corresponding to a timeperiod T_(slot). The first downlink slot comprises a control channelregion within the first OFDM symbol(s). Each sub-frame consists of agiven number of OFDM symbols in the time domain, each OFDM symbolspanning over the entire bandwidth of the component carrier.

FIG. 2 is an example illustrating further details of LTE resources. Inparticular, the smallest unit of resources that can be assigned by ascheduler is a resource block also called physical resource block (PRB).A PRB 210 is defined as N_(symb) ^(DL) consecutive OFDM symbols in thetime domain and N_(sc) ^(RB) sub-carriers in the frequency domain. Inpractice, the downlink resources are assigned in resource block pairs. Aresource block pair consists of two resource blocks. It spans N_(sc)^(RB) sub-carriers in the frequency domain and the entire 2·N_(symb)^(DL) modulation symbols of the sub-frame in the time domain. N_(symb)^(DL) may be either 6 or 7 resulting in either 12 or 14 OFDM symbols intotal. Consequently, a physical resource block 210 consists of N_(symb)^(DL)×N_(sc) ^(RB) resource elements 220 corresponding to one slot inthe time domain and 180 kHz in the frequency domain (further details onthe downlink resource grid can be found, for example, in 3GPP TS 36.211,“Evolved universal terrestrial radio access (E-UTRA); physical channelsand modulations (Release 8)”, version 8.9.0, December 2009, Section 6.2,available at http://www.3gpp.org. which is incorporated herein byreference).

The number of physical resource blocks N_(RB) ^(DL) in downlink dependson the downlink transmission bandwidth configured in the cell and is atpresent defined in LTE as being from the interval of 6 to 110 PRBs.

The data are mapped onto physical resource blocks by means of pairs ofvirtual resource blocks. A pair of virtual resource blocks is mappedonto a pair of physical resource blocks. The following two types ofvirtual resource blocks are defined according to their mapping on thephysical resource blocks in LTE downlink:

Localised Virtual Resource Block (LVRB)

Distributed Virtual Resource Block (DVRB)

In the localised transmission mode using the localised VRBs, the eNB hasfull control which and how many resource blocks are used, and should usethis control usually to pick resource blocks that result in a largespectral efficiency. In most mobile communication systems, this resultsin adjacent physical resource blocks or multiple clusters of adjacentphysical resource blocks for the transmission to a single userequipment, because the radio channel is coherent in the frequencydomain, implying that if one physical resource block offers a largespectral efficiency, then it is very likely that an adjacent physicalresource block offers a similarly large spectral efficiency. In thedistributed transmission mode using the distributed VRBs, the physicalresource blocks carrying data for the same UE are distributed across thefrequency band in order to hit at least some physical resource blocksthat offer a sufficiently large spectral efficiency, thereby obtainingfrequency diversity.

In 3GPP LTE Release 8 there is only one component carrier in uplink anddownlink. Downlink control signalling is basically carried by thefollowing three physical channels:

Physical control format indicator channel (PCFICH) for indicating thenumber of OFDM symbols used for control signalling in a sub-frame (i.e.the size of the control channel region);

Physical hybrid ARQ indicator channel (PHICH) for carrying the downlinkACK/NACK associated with uplink data transmission; and

Physical downlink control channel (PDCCH) for carrying downlinkscheduling assignments and uplink scheduling assignments.

The PCFICH is sent from a known position within the control signallingregion of a downlink sub-frame using a known pre-defined modulation andcoding scheme. The user equipment decodes the PCFICH in order to obtaininformation about a size of the control signalling region in asub-frame, for instance, the number of OFDM symbols. If the userequipment (UE) is unable to decode the PCFICH or if it obtains anerroneous PCFICH value, it will not be able to correctly decode theL1/L2 control signalling (PDCCH) comprised in the control signallingregion, which may result in losing all resource assignments containedtherein.

The PDCCH carries control information, such as, for instance, schedulinggrants for allocating resources for downlink or uplink datatransmission. A physical control channel is transmitted on anaggregation of one or several consecutive control channel elements(CCEs). Each CCE corresponds to a set of resource elements grouped toso-called resource element groups (REG). A control channel elementtypically corresponds to 9 resource element groups. A scheduling granton PDCCH is defined based on control channel elements (CCE). Resourceelement groups are used for defining the mapping of control channels toresource elements. Each REG consists of four consecutive resourceelements excluding reference signals within the same OFDM symbol. REGsexist in the first one to four OFDM symbols within one sub-frame. ThePDCCH for the user equipment is transmitted on the first of either one,two or three OFDM symbols according to PCFICH within a sub-frame.

Another logical unit used in mapping of data onto physical resources in3GPP LTE Release 8 (and later releases) is a resource block group (RBG).A resource block group is a set of consecutive (in frequency) physicalresource blocks. The concept of RBG provides a possibility of addressingparticular RBGs for the purpose of indicating a position of resourcesallocated for a receiving node (e.g. UE), in order to minimise theoverhead for such an indication, thereby decreasing the control overheadto data ratio for a transmission. The size of RBG is currently specifiedto be 1, 2, 3, or 4, depending on the system bandwidth, in particular,on N_(RB) ^(DL). Further details of RBG mapping for PDCCH in LTE Release8 may be found in 3GPP TS 36.213 “Evolved Universal terrestrial RadioAccess (E-UTRA); Physical layer procedures”, v8.8.0, September 2009,Section 7.1.6.1, freely available at http://www.3gpp.org/ andincorporated herein by reference.

Physical downlink shared channel (PDSCH) is used to transport user data.PDSCH is mapped to the remaining OFDM symbols within one sub-frame afterPDCCH. The PDSCH resources allocated for one UE are in the units ofresource block for each sub-frame.

FIG. 3 shows an exemplary mapping of PDCCH and PDSCH within a sub-frame.The first two OFDM symbols form a control channel region (PDCCH region)and are used for L1/L2 control signalling. The remaining twelve OFDMsymbols form data channel region (PDSCH region) and are used for data.Within a resource block pairs of all sub-frames, cell-specific referencesignals, so-called common reference signals (CRS), are transmitted onone or several antenna ports 0 to 3. In the example of FIG. 3, the CRSare transmitted from two antenna ports: R0 and R1.

Moreover, the sub-frame also includes UE-specific reference signals,so-called demodulation reference signals (DM-RS) used by the userequipment for demodulating the PDSCH. The DM-RS are only transmittedwithin the resource blocks in which the PDSCH is allocated for a certainuser equipment. In order to support multiple input/multiple output(MIMO) with DM-RS, four DM-RS layers are defined meaning that at most,MIMO of four layers is supported. In this example, in FIG. 3, DM-RSlayer 1, 2, 3 and 4 are corresponding to MIMO layer 1, 2, 3 and 4.

One of the key features of LTE is the possibility to transmit multicastor broadcast data from multiple cells over a synchronized singlefrequency network which is known as multimedia broadcast singlefrequency network (MBSFN) operation. In MBSFN operation, UE receives andcombines synchronized signals from multiple cells. To facilitate this,UE needs to perform a separate channel estimation based on an MBSFNreference signal. In order to avoid mixing the MBSFN reference signaland normal reference signal in the same sub-frame, certain sub-framesknown as MBSFN sub-frames are reserved from MBSFN transmission. Thestructure of an MBSFN sub-frame is shown in FIG. 4 up to two of thefirst OFDM symbols are reserved for non-MBSFN transmission and theremaining OFDM symbols are used for MBSFN transmission. In the first upto two OFDM symbols, PDCCH for uplink resource assignments and PHICH canbe transmitted and the cell-specific reference signal is the same asnon-MBSFN transmission sub-frames. The particular pattern of MBSFNsub-frames in one cell is broadcasted in the system information of thecell. UEs not capable of receiving MBSFN will decode the first up to twoOFDM symbols and ignore the remaining OFDM symbols. MBSFN sub-frameconfiguration supports both 10 ms and 40 ms periodicity. However,sub-frames with number 0, 4, 5 and 9 cannot be configured as MBSFNsub-frames. FIG. 4 illustrates the format of an MBSFN subframe.

The PDCCH information sent on the L1/L2 control signalling may beseparated into the shared control information and dedicated controlinformation. The frequency spectrum for IMT-advanced was decided at theWorld Radio Communication Conference (WRC-07) in November 2008. However,the actual available frequency bandwidth may differ for each region orcountry. The enhancement of LTE standardized by 3GPP is calledLTE-advanced (LTE-A) and has been approved as the subject matter ofRelease 10. LTE-A Release 10 employs carrier aggregation according towhich two or more component carriers as defined for LTE Release 8 areaggregated in order to support wider transmission bandwidth, forinstance, transmission bandwidth up to 100 MHz. It is commonly assumedthat the single component carrier does not exceed a bandwidth of 20 MHz.A terminal may simultaneously receive and/or transmit on one or multiplecomponent carriers depending on its capabilities.

Another key feature of the LTE-A is providing relaying functionality bymeans of introducing relay nodes to the UTRAN architecture of 3GPPLTE-A. Relaying is considered for LTE-A as a tool for improving thecoverage of high data rates, group mobility, temporary networkdeployment, the cell edge throughput and/or to provide coverage in newareas.

A relay node is wirelessly connected to radio access network via a donorcell. Depending on the relaying strategy, a relay node may be part ofthe donor cell or, alternatively, may control the cells on its own. Incase the relay node is a part of the donor cell, the relay node does nothave a cell identity on its own, however, may still have a relay ID. Inthe case the relay node controls cells on its own, it controls one orseveral cells and a unique physical layer cell identity is provided ineach of the cells controlled by the relay. At least, “type 1” relaynodes will be a part of 3GPP LTE-A. A “type 1” relay node is a relayingnode characterized by the following:

The relay node controls cells each of which appears to a user equipmentas a separate cell distinct from the donor cell.

The cells should have its own physical cell ID as defined in LTE Release8 and the relay node shall transmit its own synchronization channels,reference symbols etc.

Regarding the single cell operation, the UE should receive schedulinginformation and HARQ feedback directly from the relay node and send itscontrolled information (acknowledgments, channel quality indications,scheduling requests) to the relay node.

The relay node should appear as a 3GPP LTE compliant eNodeB to 3GPP LTEcompliant user equipment in order to support the backward compatibility.

The relay node should appear differently to the 3GPP LTE eNodeB in orderto allow for further performance enhancements to the 3GPP LTE-Acompliant user equipments.

FIG. 5 illustrates an example 3GPP LTE-A network structure using relaynodes. A donor eNodeB (d-eNB) 510 directly serves a user equipment UE1515 and a relay node (RN) 520 which further serves UE2 525. The linkbetween donor eNodeB 510 and the relay node 520 is typically referred toas relay backhaul uplink/downlink. The link between the relay node 520and user equipment 525 attached to the relay node (also denoted r-UEs)is called (relay) access link.

The donor eNodeB transmits L1/L2 control and data to the micro-userequipment UE1 515 and also to a relay node 520 which further transmitsthe L1/L2 control and data to the relay-user equipment UE2 525. Therelay node may operate in a so-called time multiplexing mode, in whichtransmission and reception operation cannot be performed at the sametime. In particular, if the link from eNodeB 510 to relay node 520operates in the same frequency spectrum as the link from relay node 520to UE2 525, due to the relay transmitter causing interference to its ownreceiver, simultaneous eNodeB-to-relay node and relay node-to-UEtransmissions on the same frequency resources may not be possible unlesssufficient isolation of the outgoing and incoming signals is provided.Thus, when relay node 520 transmits to donor eNodeB 510, it cannot, atthe same time, receive from UEs 525 attached to the relay node.Similarly, when a relay node 520 receives data from donor eNodeB, itcannot transmit data to UEs 525 attached to the relay node. Thus, thereis a sub-frame partitioning between relay backhaul link and relay accesslink.

Regarding the support of relay nodes, in 3GPP it has currently beenagreed that:

Relay backhaul downlink sub-frames during which eNodeB to relay downlinkbackhaul transmission is configured, are semi-statically assigned.

Relay backhaul uplink sub-frames during which relay-to-eNodeB uplinkbackhaul transmission is configured are semi-statically assigned orimplicitly derived by HARQ timing from relay backhaul downlinksub-frames.

In relay backhaul downlink sub-frames, a relay node will transmit todonor eNodeB and consequently r-UEs are not supposed to expect receivingany data from the relay node. In order to support backward compatibilityfor UEs that are not aware of their attachment to a relay node (such asRelease 8 UEs for which a relay node appears to be a standard eNodeB),the relay node configures backhaul downlink sub-frames as MBSFNsub-frames.

In the following, a network configuration as shown in FIG. 5 is assumedfor exemplary purposes. The donor eNodeB transmits L1/L2 control anddata to the macro-user equipment (UE1) and 510 also to the relay (relaynode) 520, and the relay node 520 transmits L1/L2 control and data tothe relay-user equipment (UE2) 525. Further assuming that the relay nodeoperates in a time-duplexing mode, i.e. transmission and receptionoperation are not performed at the same time, we arrive at anon-exhaustive entity behavior over time as shown in FIG. 6. Wheneverthe relay node is in “transmit” mode, UE2 needs to receive the L1/L2control channel and physical downlink shared channel (PDSCH), while whenthe relay node is in “receive” mode, i.e. it is receiving L1/L2 controlchannel and PDSCH from the Node B, it cannot transmit to UE2 andtherefore UE2 cannot receive any information from the relay node in sucha sub-frame. In the case that the UE2 is not aware that it is attachedto a relay node (for instance, a Release-8 UE), the relay node 520 hasto behave as a normal (e-)NodeB. As will be understood by those skilledin the art, in a communication system without relay node any userequipment can always assume that at least the L1/L2 control signal ispresent in every sub-frame. In order to support such a user equipment inoperation beneath a relay node, the relay node should therefore pretendsuch an expected behavior in all sub-frames.

As shown in FIGS. 3 and 4, each downlink sub-frame consists of twoparts, control channel region and data region. FIG. 7 illustrates anexample of configuring MBSFN frames on relay access link in situation,in which relay backhaul transmission takes place. Each subframecomprises a control data portion 710, 720 and a data portion 730, 740.The first OFDM symbols 720 in an MBSFN subframe are used by the relaynode 520 to transmit control symbols to the r-UEs 525. In the remainingpart of the sub-frame, the relay node may receive data 740 from thedonor eNodeB 510. Thus, there cannot be any transmission from the relaynode 520 to the r-UE 525 in the same sub-frame. The r-UE receives thefirst up to two OFDM control symbols and ignores the remaining part ofthe sub-frame. Non-MBSFN sub-frames are transmitted from the relay node520 to the r-UE 525 and the control symbols 710 as well as the datasymbols 730 are processed by the r-UE 525. An MBSFN sub-frame can beconfigured for every 10 ms on every 40 ms. Thus, the relay backhauldownlink sub-frames also support both 10 ms and 40 ms configurations.Similarly to the MBSFN sub-frame configuration, the relay backhauldownlink sub-frames cannot be configured at sub-frames with #0, #4, #5and #9.

Since MBSFN sub-frames are configured at relay nodes as downlinkbackhaul downlink sub-frames, the relay node cannot receive PDCCH fromthe donor eNodeB. Therefore, a new physical control channel (R-PDCCH) isused to dynamically or “semi-persistently” assign resources within thesemi-statically assigned sub-frames for the downlink and uplink backhauldata. The downlink backhaul data is transmitted on a new physical datachannel (R-PDSCH) and the uplink backhaul data is transmitted on a newphysical data channel (R-PUSCH). The R-PDCCH(s) for the relay nodeis/are mapped to an R-PDCCH region within the PDSCH region of thesub-frame. The relay node expects to receive R-PDCCH within the regionof the sub-frame. In time domain, the R-PDCCH region spans theconfigured downlink backhaul sub-frames. In frequency domain, theR-PDCCH region exists on certain resource blocks preconfigured for therelay node by higher layer signalling. Regarding the design and use ofan R-PDCCH region within a sub-frame, the following characteristics havebeen agreed so far in standardization:

R-PDCCH is assigned PRBs for transmission semi-statically. Moreover, theset of resources to be currently used for R-PDCCH transmission withinthe above semi-statically assigned PRBs may vary dynamically, betweensub-frames.

The dynamically configurable resources may cover the full set of OFDMsymbols available for the backhaul link or may be constrained to theirsub-set.

The resources that are not used for R-PDCCH within the semi-staticallyassigned PRBs may be used to carry R-PDSCH or PDSCH.

In case of MBSFN sub-frames, the relay node transmits control signals tothe r-UEs. Then, it can become necessary to switch transmitting toreceiving mode so that the relay node may receive data transmitted bythe donor eNodeB within the same sub-frame. In addition to this gap, thepropagation delay for the signal between the donor eNodeB and the relaynode has to be taken into account. Thus, the R-PDCCH is firsttransmitted starting from an OFDM symbol which, within the sub-frame, islate enough in order for a relay node to receive it.

The mapping of R-PDCCH on the physical resources may be performed eitherin a frequency distributed manner or in a frequency localised manner.

The interleaving of R-PDCCH within the limited number of PRBs canachieve diversity gain and, at the same time, limit the number of PRBswasted.

In non-MBSFN sub-frames, Release 10 DM-RS is used when DM-RS areconfigured by ENodeB. Otherwise, Release 8 CRS are used. In MBSFNsub-frames, Release 10 DM-RS are used.

R-PDCCH can be used for assigning downlink grant or uplink grant for thebackhaul link. The boundary of downlink grant search space and uplinkgrant search space is a slot boundary of the sub-frame. In particular,the downlink grant is only transmitted in the first slot and the uplinkgrant is only transmitted in the second slot of the sub-frame.

No interleaving is applied when demodulating with DM-RS. Whendemodulating with CRS, both REG level interleaving and no interleavingare supported.

Based on the above agreement, there are basically three differentoptions for configuration of the R-PDCCH search space:

Frequency localised non-interleaved R-PDCCH,

Frequency distributed non-interleaved R-PDCCH, and

REG-level interleaved R-PDCCH.

For the REG-level interleaved R-PDCCH, the Release 8 PDCCH search spacescheme will be reused within the semi-statically configured PRBs forR-PDCCH (the so-called R-PDCCH virtual bandwidth). For thenon-interleaved R-PDCCH, the Release 8 PDCCH search space concept ofrandomizing the positions of PDCCH candidates for different aggregationlevels across the whole bandwidth could theoretically be applied, butwould not facilitate the benefit that the candidates can be in positionswhich can be freely assigned by the eNodeB. This would, in turn, make itimpossible to exploit the full frequency-selective scheduling benefitfor the control channel.

SUMMARY OF THE INVENTION

In view of the above, the aim of the present invention is to provide anefficient scheme for configuring a search space in which controlinformation, which may contain uplink and downlink grants (orassignments) for a shared channel, can be signalled to a receiver, orparticularly to a relay node.

This is achieved by the features of independent claims.

Advantageous embodiments of the invention are subject to the dependentclaims.

It is the particular approach of the present invention to provide asearch space configuration with localised lower aggregation levelcandidates and at least one distributed candidate of a higheraggregation level.

In accordance with an aspect of the present invention, a method isprovided for receiving control data within a subframe in a multi-carriercommunication system, the method comprising the following steps to beperformed at a receiving node: receiving a sub-frame from a transmittingnode, wherein the sub-frame is logically divided into physical resourceblocks and comprises a plurality of control channel elements; andperforming a blind detection for a control information within apredefined search space of the sub-frame. Said search space is logicallysubdivided into candidates for performing blind detection, eachcandidate comprising one or more aggregated control channel elements,wherein at least one candidate having a first number of aggregatedcontrol channel elements consists of control channel elements placedadjacent in frequency, and at least one candidate having a second numberof aggregated control channel elements, greater than the first number,consists of control channel elements at least partially distributed infrequency.

In accordance with another aspect of the present invention, a method isprovided for transmitting control information for at least one receivingnode within a subframe of a multi-carrier communication system, themethod comprising the following steps to be performed at thetransmitting node: mapping control information for a receiving node ontoa predefined search space in a subframe, the search space comprisingresources on which receiving node is to perform a blind detection,wherein said search space is logically subdivided into candidates forblind detection, each candidate comprising one or more aggregatedcontrol channel elements, wherein at least one candidate having a firstnumber of aggregated control channel elements consists of controlchannel elements placed adjacent in frequency, and at least onecandidate having a second number of aggregated control channel elements,greater than the first number, consists of control channel elements atleast partially distributed in frequency; and transmitting the subframeto the at least one receiving node.

In accordance with another aspect of the present invention, a receivingapparatus is provided for receiving a control information within asubframe of a multi-carrier communication system, the apparatuscomprising: a receiving unit for receiving a sub-frame from atransmitting node, wherein the sub-frame comprises a plurality ofcontrol channel elements; and a detecting unit for performing a blinddetection for a control information within a predefined search space ofthe sub-frame, wherein said search space is logically subdivided intocandidates for which the blind detection is to performed, each candidatecomprising one or more aggregated control channel elements, wherein atleast one candidate having a first number of aggregated control channelelements consists of control channel elements placed adjacent infrequency, and at least one candidate having a second number ofaggregated control channel elements, greater than the first number,consists of control channel elements at least partially distributed infrequency.

In accordance with another aspect of the present invention atransmitting apparatus is provided for transmitting control informationfor at least one receiving node within a subframe of a multi-carriercommunication system, the apparatus comprising: a mapping unit formapping control information for a receiving node onto a predefinedsearch space in a subframe, the search space comprising resources onwhich receiving node is to perform a blind detection, wherein saidsearch space is logically subdivided into candidates for blinddetection, each candidate comprising one or more aggregated controlchannel elements, wherein at least one candidate having a first numberof aggregated control channel elements consists of control channelelements placed adjacent in frequency, and at least one candidate havinga second number of aggregated control channel elements, greater than thefirst number, consists of control channel elements at least partiallydistributed in frequency; and a transmitting unit for transmitting thesub-frame to the at least one receiving node.

Accordingly, the robustness and resilience against fading dip isincreased for the higher aggregation level candidates even if localisedcandidates are configured. In other words, the present inventionsuggests distributing at least partially at least one of the higheraggregation level candidates in the frequency domain. Here the term “todistribute” refers to mapping the candidate on control channel elementswhich are not all mapped on sequentially adjacent physical resourceblocks in frequency. Thus, at least one other physical resource blockseparates at least two portions of the control channel elements of thecandidate.

Advantageously, the first number of aggregated control channel elementsis 1 or 2 and the second number of aggregated control channel elementsis 4 or 8. Preferably, one candidate with the second number ofaggregated control channel elements is distributed over single controlchannel element and/or candidate with 8 aggregated control channelelements is distributed over portions with size of 2 control channelelements, where a portion generally resembles two or more sequentiallyadjacent physical resource blocks. These particular numbers areespecially advantageous with regard to the LTE/LTE-A system, which nowsupport aggregation levels 1, 2, 4, and 8. The selection of thesenumbers enables localization of aggregation level 2 and distributes atleast one candidate of aggregation level 4 and/or 8, which increase thedetection robustness for these distributed candidates. However, thepresent invention is not limited to these numbers. Depending on thesystem to be deployed in, the aggregation levels of 3, 5, 6, etc. mayalso be supported. In general, the selection of the aggregation levelsthe candidates of which are to be distributed may be performed withregard to the frequency band in which the candidates are defined and theexpected fading characteristics.

In accordance with an embodiment of the present invention, the controlchannel elements correspond to the Control Channel Elements (CCEs) asdefined in 3GPP LTE/LTE-A. In accordance with another embodiment of thepresent invention, the control channel elements correspond to physicalresource blocks of 3GPP LTE/LTE-A. However, the present invention is notlimited thereto and the control channel elements of the presentinvention may be any resources defined in the time/frequency domain.

Advantageously, at least one candidate with the second number ofaggregated control channel elements is mapped on physical resourceblocks adjacent in frequency (i.e. in a localised manner) and at leastone other candidate with the second number of aggregated control channelelements is mapped on physical resource blocks distributed in frequency.A localised manner is advantageously employed if the fading and/or noiseand interference characteristic is flat in frequency domain, such thatadjacent physical resource blocks have a very high probability of havingvery similar channel characteristics. For most aspects, the mostimportant channel characteristics include amplitude and phase changesdue to signal fading and propagation, noise, interference, and/orfrequency shift. Consequently, with a good knowledge of the channelcharacteristic, the channel capacity can be very efficiently exploitedby using a localised manner by selective using the best channelresource(s). Conversely, without such knowledge or with an inaccurateknowledge, it is beneficial to use distributed channel resources inorder to try to hit at least some resources where the channel state isgood (i.e. in a more or less random fashion).

Preferably, the distributing of the candidate with the second number ofaggregated control channel elements in frequency is performed by mappingsaid candidate onto portions with a third number of control channelelements, the portions being further mapped to the frequency in adistributed way, i.e. separated from each other by at least one physicalresource block. The third number is lower than the second number. Inparticular the control channel elements of a candidate may bedistributed to the positions in frequency corresponding to positionscandidates of a lower aggregation level than the second number. Forinstance, a candidate of aggregation level 4 or 8 may be distributed inmultiple portions, where a portion consists of control channel elementsmapped to two adjacent physical resource blocks, and where at least onesuch portions on its own would constitute an aggregation level 2candidate, or even each such portion would constitute an aggregationlevel 2 candidate. However, the distribution of the portions does notneed to be performed over positions of lower level candidates. Anycontrol channel element positions in frequency may be taken. Using thesame positions has benefits to pack the candidates in as few physicalresource blocks as possible, thereby minimising the amount of physicalresource blocks that cannot be used for shared data channeltransmission. Using different positions has benefits to allow thetransmission of different candidates with different aggregation levelswithout affecting each other, i.e. the transmission of a candidate witha lower aggregation level does not block the transmission of a candidatewith a higher aggregation level, and vice versa. It is advantageous todistribute the portions possibly far from each other in the frequencydomain and/or possibly far from other candidates to achieve higherdiversity.

Alternatively, the candidate may be distributed to a plurality of singlecontrol channel elements mapped to physical resource blocks separatedfrom each other in frequency by at least one physical resource block.

Advantageously, the positions of distributed candidates with the secondnumber of aggregated control channel elements for a first receiving nodeare different to positions of distributed candidates with the secondnumber of aggregated control channel elements for a second receivingnode. This enables an efficient mapping without blocking the resourcesfor the receiving-node-dedicated signalling.

According to an embodiment of the present invention, candidates with ahigher number of aggregated control channel elements localised onadjacent frequencies are located in frequency without overlapping withcandidates with a lower number of aggregated control channel elements.Such an arrangement is particularly advantageous since it enablesefficient utilization of resources without blocking. This means, forinstance, that assignment of level 1 candidates does not blockassignment of a level 2 candidate but rather, both may be assigned tothe same or different receiving nodes.

In particular, according to an embodiment of the present invention, thecontrol channel elements are mapped on physical resource blocks and afourth number of physical resource blocks form a resource block group,the candidates being mapped to control channel elements which areincluded in physical resource blocks of a resource block group.Candidates with a lower number of control channel elements arelocalised, mapped on adjacent (in frequency) physical resource blocksstarting with the first physical resource block of a resource blockgroup, and candidates with a higher number of aggregated control channelelements are localised, mapped on adjacent physical resource blocksending on the last physical resource block of the resource block group,where for candidates with a higher number of aggregated control channelelements than the size of a resource block group, a candidate fills afirst resource block group completely and an adjacent second resourceblock group at least partly. Such a mapping is likely to avoidoverlapping of candidates of different aggregation levels and is, thus,efficient with respect to the resource utilization. The fourth numbermay be, for instance 2, 3, or 4. However, in general any other numbermay be supported as well. The fourth number equal to 1 means that aphysical resource block alone corresponds to a resource block group.

Alternatively, in order to avoid the overlapping of candidates havingdifferent aggregation levels, candidates with a higher number ofaggregated control channel elements mapped localised on adjacentphysical resource blocks are located in frequency in physical resourceblocks shifted with respect to the positions of the candidates with alower number of aggregated control channel elements, each loweraggregation candidate being mapped localised on adjacent physicalresource blocks. In particular, the shift is sufficient so that thecandidates do not overlap, for instance it may have the size of thelower number of aggregated control channel elements, meaning that thehigher-level candidate starts in a physical resource block next to thelower-level candidate.

Preferably, the receiving node is a relay node, transmitting node is adonor eNodeB in a 3GPP LTE(-A) based system, and the physical downlinkcontrol channel is R-PDCCH without physical resource block interleavingin time domain. However, alternatively or in addition, a regular mobileterminal may act as the receiving node, in order to benefit from alocalised control information transmission.

In accordance with another embodiment of the present invention, thehigher level candidates of different receiving nodes are distributed topositions that do not overlap. The positions of localised candidateswith the first number of aggregated physical resource blocks may beidentical for a first receiving node and a second receiving node.

In accordance with another aspect of the present invention, a computerprogram product comprising a computer-readable medium having acomputer-readable program code embodied thereon is provided, the programcode being adapted to carry out the present invention.

The above and other objects and features of the present invention willbecome more apparent from the following description and preferredembodiments given in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic drawing showing the general structure of asub-frame on a downlink component carrier defined for 3GPP LTE release8;

FIG. 2 is a schematic drawing showing an exemplary downlink componentcarrier of one of two downlink slots of a sub-frame defined for 3GPP LTErelease 8;

FIG. 3 is a schematic drawing illustrating the structure of a non-MBSFNsub-frames and a physical resource block pair thereof defined for 3GPPLTE release 8 and 3GPP LTE-a release 10;

FIG. 4 is a schematic drawing illustrating a structure of MBSFNsub-frames and a physical resource block pair thereof defined for 3GPPLTE Release 8 and 3GPP LTE-A Release 10.

FIG. 5 is a schematic drawing of an exemplary network configurationincluding a donor eNodeB, a relay node, and two user equipments;

FIG. 6 is a schematic drawing illustrating exemplary behaviour of thedonor eNodeB, the relay node and the two user equipments of FIG. 3 withrespect to the operation in transmission mode and reception mode;

FIG. 7 is a schematic drawing illustrating an example of a structure ofa relay backhaul downlink sub-frame configurations used in 3GPP LTE-ARelease 10;

FIG. 8 is a schematic drawing illustrating an example of a localisedsearch space for uplink and downlink grants with candidates of variousaggregation levels;

FIG. 9 is a schematic drawing illustrating an example of a distributedsearch space for uplink and downlink grants with candidates of variousaggregation levels, and the relation between virtual resource blocks andphysical resource blocks in a distributed mapping case;

FIG. 10 is a schematic drawing illustrating an example of mapping theaggregation level 1, 2 and 4 candidates with resource block groups ofsize 4 in accordance with an embodiment of the present invention;

FIG. 11 is a schematic drawing illustrating an example of a search spacefor different sizes of RGB with candidates aligned with the ends ofrespective RBGs;

FIG. 12 is a schematic drawing illustrating an example of a search spacefor different sizes of RGB with some candidates shifted, where possible,with respect to the beginning of an RBG;

FIG. 13 is a schematic drawing illustrating an example of a search spacefor different sizes of RBG with aligning of candidates to the start andto end of the respective group;

FIG. 14 is a flowchart illustrating an operation of a method accordingto an embodiment of the present invention; and

FIG. 15 is a schematic drawing illustrating a network with two relaynodes and a donor eNodeB.

DETAILED DESCRIPTION

The following paragraphs will describe various embodiments of thepresent invention. For exemplary purposes only, most of the embodimentsare outlined in relation to an OFDM downlink radio access schemeaccording to 3GPP LTE (Release 8) and LTE-A (Release 10) mobilecommunication systems discussed in the Technical Background sectionabove. It should be noted that the invention may be advantageously usedfor example in connection with a mobile communication system such as3GPP LTE (Release 8) and LTE-A (Release 10) communication systemspreviously described, but the invention is not limited to its use inthis particular exemplary communication network. The aspects of theinvention described herein may be inter alia used for defining thesearch spaces for uplink and downlink control information (R-PDCCH),mainly carrying assignments and grants for receivers such as relay nodesor UEs in a 3GPP LTE-A (Release 10) communication systems and forproviding an effective resource utilization in terms of R-PDCCH databeing mapped to a downlink search space (in particular to individualresource blocks and resource block groups thereof). The explanationsgiven in the Technical Background section above are intended to betterunderstand the mostly 3GPP LTE (Release 8) and LTE-A (Release 10)specific exemplary embodiments described herein and should not beunderstood as limiting the invention to the described specificimplementations of processes and functions in the mobile communicationnetwork. Specifically, it can be applied to the communication betweentwo non-mobile nodes of a communication network, such as between astationary eNodeB and a stationary relay node.

In general, the present invention provides a search space configurationwith localised lower aggregation level candidates and at least onedistributed candidate of a higher aggregation level.

In particular, the channel structure for carrying control informationfor at least one receiving node within a sub-frame of a communicationsystem is as follows. The search space within a sub-frame is formed by aplurality of control channel elements. The search space is logicallysubdivided into candidates for blind decoding by the at least onereceiving node. Each candidate includes one or more aggregated controlchannel elements, wherein at least one candidate has a first number ofaggregated control channel elements and these control channel elementsare transmitted adjacently or very closely to each other in thefrequency domain. Another at least one candidate has a second number ofaggregated control channel elements, greater than the first number, andconsists of control channel elements at least partially distributed infrequency, where the level of distribution is preferably in the order ofmore than two transmission units in the frequency domain. Thetransmission unit is, for instance, a PRB or a control channel element.The distribution level here means the distance between two candidateportions.

Such a search space is particularly advantageous for communicationsystems in which the quality of channel may vary rapidly and in whichthe signal may suffer from sudden fading dips, or where the channel isselective in the frequency domain but the knowledge about the channel isinaccurate. This is especially the case for wireless systems with mobileand/or static nodes using multiple carriers. An example therefor is a3GPP LTE based system and the present invention may readily be employedin such a system. However, the present invention is not limited to suchkinds of systems and may be used in any communication system withmultiple carriers to configure a search space for any information to beblindly decoded. The information to be blindly decoded is typicallycontrol information which enables a node to access further shared ordedicated control resources.

In these terms, the receiving node may be any node capable of receivingcontrol information in such a multi-carrier communication system. Forinstance, the receiving node may be a user terminal which may be mobileor fixed. Such a user terminal may, but does not necessarily, work as arelay node for other terminals. Alternatively, the receiving node may bea separate relay node. Such a relay node may be fixed (for instance inorder to increase a coverage of a base station) or it may be mobile.However, the receiving node may also be any other node such as a basestation or other network node. Similarly, the transmitting node may beany network node, for instance, a base station, or a relay node, or auser terminal. In the context of LTE, in an advantageous embodiment ofthe present invention, the transmitting node is an eNodeB, inparticular, a donor eNodeB and the receiving terminal is a relay node.This embodiment is particularly advantageous since the search space forR-PDCCH has not been standardized so far. However, in another embodimentof the present invention the transmitting node may be a relay node andthe receiving node may be a user equipment or vice versa. Bothtransmitting and receiving nodes may also be relay nodes or userterminals.

Regarding the control information, this may be any control informationdirected to the receiving node. In particular, the control informationmay indicate the location of further resources, dedicated or shared, forthe transmission or reception of data by the receiving terminal. Inparticular, the control information may include downlink or uplinkgrants. Alternatively, it can contain commands for power control, or fortriggering certain receiver actions such as emission of channel soundingsignals, or the deactivation of a communication channel or service.

In general, the search space consists of candidates for blind decoding.Candidates are in this context subsets of physical system resources. Thebasic resource element for transmission of the control information isthe control channel element. Each candidate may include one controlchannel element or more aggregated control channel elements. The controlchannel elements may correspond to a particular frequency range (one ormore carriers of the multi-carrier system) and have a predefined timeduration. Here, the control channel element represents a smallestphysical resource portion which is addressable for the transmission ofthe control information. In context of LTE, the control channel elementmay be, for instance, a CCE or a physical resource block, with thepossible exception of resource elements that are occupied byindispensable signals such as reference symbols, as is exemplarily shownin FIG. 3 by the resource elements carrying common reference symbols(CRS) or demodulation reference symbols (DM-RS). It can further compriseonly a subset of OFDM symbols within a slot as shown in FIG. 4, whereonly the second part of the first slot is usable for carrying controlchannel elements.

The search space according to the present invention is configured toinclude both localised and distributed candidates. Localised candidatesare candidates the control channel elements of which are located in asequence adjacently to each other in frequency domain. Distributedcandidates are not localised, meaning that their control channelelements are located in the frequency domain separated from each otherby at least one control channel element not belonging to the samecandidate. The control channel elements of the candidate may bedistributed over single non-adjacent control channel elements (separatedfrom each other in frequency by at least one physical resource block). Acandidate may also be distributed partially, which means that thecandidate is subdivided into portions of more than one adjacent controlchannel elements, and the portions are further distributed in frequency.The portions may, but do not need to have, the same size.

Global deployment of mobile communication systems brought a requirementof providing a widest possible coverage and to support terminals withhigh mobility. To facilitate this, the concept of relay nodes has beenstandardized. Relay nodes working in a frequency band common for accesslink (link to and from a terminal) and relay (backhaul) link (link toand from a network node such as an eNodeB) provide advantages includingreduced costs and can be more easily deployed. However, as describedabove, such relay nodes typically work in a time division mode, meaningthat they cannot exchange data at the same time on relay access andbackhaul link. Consequently, the relay node resources have to be sharedfor transmitting and/or receiving of data to/from a network node andto/from a terminal.

According to an exemplary embodiment of the present invention, a searchspace is provided for channel carrying resource assignments, inparticular, signalling of resource assignments on and for the backhaullink. The search space includes physical resources defined in terms oftime and frequency resources, which carry uplink and/or downlink grantsand which are typically received by a relay node from a network node.Configuration of such a search space is advantageously reconfigurable,meaning that the position of the search space may be set and signalledfrom the network node to the relay node. In order to maintain the systemefficient, such a signalling should preferably require as low bandwidthas possible.

In 3GPP LTE, the resources may be allocated in terms of physicalresource blocks (PRB). Some control channels allow for assigning evensmaller resource portions. For instance, the PDCCH control channelregion within a sub-frame consists of a set of control channel elements(CCEs). A PDCCH can aggregate 1, 2, 4 or 8 CCEs. Similarly, R-PDCCHshall likely support aggregation levels 1, 2, 4, and 8. The aggregationmay be over CCEs or over physical resource blocks. In the following,examples are described with aggregation of the physical resource blocks.However, all these examples are applicable also to aggregation of CCEsas a unit of physical resources.

Each relay node monitors a set of R-PDCCH candidates of any aggregationlevels for control information in every non-DRX subframe. Monitoringrefers to attempting to decode each of the R-PDCCHs in the set accordingto all monitored formats, i.e. blind decoding. Blind decoding isdescribed for UE receiving a PDCCH in 3GPP TS 36.213 “Evolved Universalterrestrial Radio Access (E-UTRA); Physical layer procedures”, v8.8.0,September 2009, Section 9.1.1, freely available athttp://www.3gpp.org/and incorporated herein by reference). According tothe present specifications for UE-specific PDCCH, the search space mayinclude six candidates of aggregation level 1 and 2 and two candidatesof aggregation levels 4 and 8. The number of candidates also specifiesthe number of blind decodings the terminal has to perform.

FIG. 8 illustrates an example of a localised search space configurationfor R-PDCCH assuming a similar configuration.

Accordingly, an R-PDCCH search space of R-PDCCH aggregation level L isconfigured by a set of virtual resource block (VRB) indices {n_(L,1),n_(L,2), . . . , n_(L,M(L))} and the R-PDCCH resource allocation type(distributed or localised VRB mapping). The configuration may besignalled, for instance, by RRC signalling. The same configuration maybe applicable to both first and second slot, carrying the downlink anduplink grants, respectively. For such a localised search space, theindex of VRB equals to the index of PRB. Therefore, L consecutive PRBsconstitute a valid R-PDCCH candidate. The starting positions of eachcandidate for each aggregation level are signalled from a donor eNodeBto a relay node. Similarly to PDCCH configuration (cf., for instance,3GPP TS 36.213 “Evolved Universal terrestrial Radio Access (E-UTRA);Physical layer procedures”, v8.8.0, September 2009, Section 9.1,available on http://www.3gpp.org/, and incorporated herein byreference), FIG. 8 assumes six candidates for aggregation level 1 and 2,and two candidates for aggregation level 4. The index of the candidatefor each aggregation level is also illustrated by a different hatching.

FIG. 9 illustrates an example of distributed search space configurationfor R-PDCCH assuming that the VRB to PRB mapping follows the rules ofRelease 8 DVRB mapping, where for each aggregation level (AL) the upperrow denotes the VRB, while the lower row denotes the corresponding PRB.

In the localised search space as shown in FIG. 8, adjacent PRBs areaggregated. However, such a localisation is detrimental in case that thePRBs are in a fading dip or in case when the interference increasessubstantially especially for higher aggregation levels, or if theknowledge of the channel at the transmitting node is inaccurate. This isbecause the higher aggregation levels, such as aggregation level 4 and8, only have two candidates for each aggregation level. These candidatesare more affected by a bad channel condition since the probability thatall candidates are facing a bad channel condition is higher. Thus, therelay node may even loose connection to the donor eNodeB, which, on theother hand, may result in terminals attached to the relay node loosingtheir connection to the network. Since in case of only a localisedsearch space, all candidates of different aggregation levels are mappedin localised way, the robustness with respect to fading dip or channelknowledge inaccuracy is reduced especially for higher aggregationlevels, such as aggregation level 4 or 8.

Furthermore, as can be seen in FIG. 8, the starting positions ofcandidates for aggregation levels 1, 2 and 4 overlap. It means that ifPRB#0 is used for R-PDCCH of aggregation level 1, PRB#0 and #1 cannot beused for R-PDCCH of aggregation level 2 for another relay node. Ingeneral, in case of non-interleaved R-PDCCH using CRS for demodulation,it is better to allocate R-PDCCHs within one resource block group (RBG)in order to reduce the number of RBGs that are occupied by R-PDCCHs.However, due to overlapping of candidates of different aggregationlevels, multiple R-PDCCHs cannot be efficiently allocated in one RBG inorder to reduce the number of RBGs occupied by R-PDCCHs in case of CRSnon-interleaved R-PDCCH.

In order to provide higher robustness against fading or channelknowledge inaccuracy, according to the present invention, in case ofconfigured localised search space, at least part of the candidates forblind decoding for the larger (largest) aggregation size is distributedover the candidates of lower level aggregation size. The lower levelaggregation sizes are preferably level 1 and/or 2. The distribution maybe performed for levels 1 and/or 2. For some systems, in which anothersizes of candidates are enabled, the distribution may be performed overany aggregation level candidate such as 3 or 4 or 5, etc.

In its last three lines, FIG. 10 illustrates an example of mapping forcandidates of aggregation level 4 (denoted as AL4). Accordingly, thefirst two aggregation level-4 candidates are localised in the frequencydomain and are mapped to resource block group #1 and resource blockgroup #4. The third candidate 1040 is distributed over candidates ofaggregation level 1. In this case, the candidate number 3 is distributedinto the first physical resource block of the respective resource blockgroups #1, #2, #4 and #6. The fourth candidate 1030 is distributed overcandidates of aggregation level 2. In particular, the fourth candidateis located at the end of the resource block group #1 and the resourceblock group #5 in portions of two physical resource blocks. Thus, thesearch space of FIG. 10 includes a distributed part of the search space1020 formed by the third and fourth candidates with aggregation levelfour. The search space further includes a localised part of 1010 formedby the first and the second candidates with aggregation level four, bysix candidates of aggregation level 2 and by another 6 candidates ofaggregation level 1.

It should be noted that the way of distributing the third and the fourthcandidates in FIG. 10 is only an example. The third level-4 candidatemay alternatively be distributed over other level-1 candidates, forinstance, over the first physical resource blocks of RBG#3 or RBG#3.

Alternatively, the distribution does not necessarily have to beperformed over lower aggregation level candidates. In general, the thirdlevel-4 candidate may be distributed over any physical resource blocks(control channel elements) of the RBGs, or even be irrespective of theRBG definition. The distribution of higher level candidates provideshigher robustness with respect to frequency selective fading. Therefore,it is particularly advantageous to distribute the higher levelcandidates to physical resource blocks possibly distanced from eachother.

Similarly, the 2-PRB large portions of fourth level-4 candidate in FIG.10 may be distributed in to any of the six RBGs. In particular, theportions may be placed in RBG#1 and RBG#6, or to RBG#2 and RBG#6, or toany other RBG# combinations. However, the more distanced the portionsare from each other, the higher the probability that at least one ofthem is not in fading. It is also advantageous to map differentcandidates of the same aggregation level and for the same receiving nodeon possibly different frequencies. The fourth level-4 candidate alsodoes not need to be mapped over level-2 candidate positions. A similareffect may be achieved by mapping its portions to the second and thirdphysical resource block of the respective groups. The portions may alsobe mapped to the first two physical resource blocks.

Thus, FIG. 10 illustrates an example of a search space with localisedlower aggregation level candidates (level-1, level-2 candidates), andwith higher aggregation level candidates (level-4) localised anddistributed. However, the present invention is not limited thereto and,alternatively, all higher aggregation level candidates may bedistributed. Alternatively, a single localises and three distributedlevel-4 candidates may be employed or vice versa.

The particular configuration of the search space should advantageouslybe designed with regard to the particular deployment scenario such asthe expected or assumed channel characteristics, which can furtherdepend on channel state feedback from the receiver. The channelcharacteristics shall depend on whether the transmitting/receiving nodesare mobile or static, their distance and location, etc. It shall furtherdepend on the frequency band used. For different deployment scenariosand systems, other particular configuration may be more suitable asshall be understood by a person skilled in the art.

The present invention provides a configuration of a search space forblind decoding of control information. In accordance with an embodimentof the present invention, such a search space is fixedly defined in thespecification of the communication system. According to anotherembodiment of the present invention, the search space is configurablestatically and may be received within system information on broadcastcontrol channel(s) of the communication system. In accordance with stillanother embodiment of the present invention, the search spaceconfiguration may be semi-statically set with higher layer signalling,such as Radio Resource Control (RRC) protocol signalling in 3GPP LTE.Alternatively, the search space may be configured dynamically forsub-frames.

Apart of the distribution of some higher aggregation level candidates,FIG. 10 also shows localised mapping of lower aggregation levelcandidates in accordance with another advantageous embodiment of thepresent invention. Accordingly, the overlapping of candidates ofdifferent aggregation levels is avoided. This is illustrated in FIG. 10for aggregation levels 1 (denoted AL1) and 2 (denotes AL2). Inparticular, the exemplary 6 candidates of aggregation level 1 are placedin the first physical resource block of each resource block group whichmeans in physical resource blocks 0, 4, 8, 12, 16 and 20. The exemplary6 candidates of aggregation level 2 are also placed in each resourceblock group. However, in order to avoid overlapping with the candidatesof aggregation level 1, the candidates of aggregation level 2 are mappedon the last two physical resource blocks of each resource block group.This mapping of aggregation level 1 and aggregation level 2 candidateson resource block groups provides a more efficient and more flexiblepossibility of defining a search space. In contrast to the exampledescribed with reference to FIG. 8, the candidates in FIG. 10 ofaggregation level 2 and aggregation level 1, mapped to the same resourceblock group, may be still assigned to different receiving nodes (forinstance relay nodes).

The entire example of FIG. 10 illustrates a search space which is bothlocalised and distributed. In particular, aggregation level candidates 1and 2 are localised and aggregation level 4 candidates are distributedand localised. It should be noted that FIG. 10 only exemplifies thepossibilities of distributing aggregation level 4 candidates. Othercombinations are possible, for instance, all aggregation level 4candidates may be distributed to single physical resource blocks orportions of a plurality of resource blocks. Alternatively, there maybeonly two aggregation level 4 candidates, only one of which isdistributed, for instance, over candidates of aggregation level 2.

In FIG. 10 the candidates of different aggregation levels are mappedonto the same resource block group with as little overlapping aspossible. In context of the 3GPP LTE backhaul link, the R-PDCCHcandidates of aggregation level 1 and 2 are mapped onto differentphysical resource blocks within one resource block group. In thisexample resource block group size is equal to 4 physical resourceblocks. R-PDCCH of aggregation level 1 can be multiplexed with R-PDCCHof aggregation level 2 within one resource block group. This is usefulin particular when multiple relay nodes share the same search space andCRS non-interleaved R-PDCCH is configured for these relay nodes.

The search space according to this embodiment of the present inventionprovides several benefits. This concept allows for dynamic switchingbetween localised and distributed aggregation in which the candidates ofhigher aggregation level are distributed whereas the candidates of loweraggregation levels are localised. This helps to avoid loosing connectionin case of sudden deteriorations of the channel and prevents a receiverto fall-back to an initial attachment procedure, for example by beingable to re-configure the search space to occupy different physicalresource blocks, or to change to a distributed mode method such as shownin FIG. 8, leading to an improved efficiency of backhaul link and to abetter service for terminals attached to the relay nodes. Moreover, therobustness of aggregation level 4 and 8 candidates is increased. This isimportant in particular in schemes in which there is lower number ofhigher aggregation level candidates since in such a case the probabilityof such candidates to being in a fading dip is rather high. In addition,avoiding of overlapping between candidates of different aggregationlevels prevent blocking among blind decoding candidates of localisedlower aggregation sizes and provides more efficient mapping of thecontrol channel on physical resources. The above described search spaceconfigurations also maintain the property of minimizing the number ofresource block groups occupied by search space and R-PDCCH allocation,since it compactly maps the candidates of different aggregation levelson the same resource block groups.

FIGS. 11, 12, 13, show examples for illustration of candidates, thecontrol channel elements of which they are composed by means ofaggregation, and their locations with respect to physical resourceblocks and RBG sizes. Generally, the first line shows candidates ofaggregation level 1, under that the candidates for aggregation level 2,under that the candidates for aggregation level 4, followed by two linesfor aggregation level 8. The different RBGs are illustrated byalternating white and gray background.

FIG. 11 illustrates further examples of implementing the presentinvention for different sizes on the resource block group. Inparticular, FIG. 11, part (a) shows an example in which the resourceblock group size is equal to 2 physical resource blocks and candidatesof aggregation levels 1, 2, 4 and 8 are mapped thereto. The candidatesof aggregation level 1 (such as, for instance, the first candidate 1101)are mapped onto each first physical resource block in each group. Thereare 6 aggregation level 1 candidates. The 6 aggregation level 2candidates are also mapped to each of the resource block groups so thatthey indeed overlap with the candidates of aggregation level 1. However,a compact mapping which requires only 6 resource block groups can beachieved. In context of LTE, the configurable number of PRBs for RBGwith size 2 is between 11 and 26 PRBs. For RBG of size 3, there are 27to 63 PRBs. For RBG of size 4, there are 64 to 110 PRBs. More details tothe possible configurations can be found in 3GPP TS 36.213 “EvolvedUniversal terrestrial Radio Access (E-UTRA); Physical layer procedures”,v8.8.0, September 2009, Section 7.1.6.1, as already mentioned above.

The two candidates of aggregation level 4 are mapped onto the first 4resource block groups each candidate covering two resource block groups.Finally, the two candidates of aggregation level 8 are mapped asfollows: the first one in a localised manner and the second one in adistributed manner. In particular, the first aggregation level 8candidate covers first 4 resource block groups. The second aggregationlevel 8 candidate is distributed over portions of 4 physical resourceblocks which is in this example a first portion covering the first tworesource block groups and a second portion covering the last resourceblock groups 5 and 6. In FIGS. 11, 12, and 13, the candidates areillustrated as dashed ellipses with the candidate number writtentherein. The size of the ellipse corresponds to the aggregation level ofthe candidate for localised candidates. For distributed candidates, theellipses illustrate the candidate's portions, which are connected with ahorizontal line. For example, FIG. 11 (a) shows the second candidate foraggregation level 8 in the bottom row aggregating two portions each ofsize 4 control channel elements, both portions being distributed by 4PRBs.

FIG. 11, part (b) illustrates an example in which the size of resourceblock group is three physical layer blocks. In particular, thecandidates of aggregation level 1 are mapped on the first physicalresource block of each of the 6 resource block groups. The candidates ofaggregation level 2 are mapped to the last two physical resource blocksof each resource block group. As can be seen, with such a mapping,candidates of aggregation level 1 do not overlap with candidates ofaggregation level 2. The two candidates of aggregation level 4 arelarger than the resource block group itself and are therefore mappedover two neighbouring resource groups. In particular, the firstaggregation level 4 candidate 1102 is mapped on the last physicalresource block of the first resource block group and on all physicalresource blocks of the second resource block group. The second candidateof aggregation level 4 is mapped similarly, on the next availableresource block groups #3 and #4. The first candidate of aggregationlevel 8 is localised and mapped similarly to the mapping of level 4candidates namely by aligning on the end of the first possible resourceblock group #3. The RBG#3 is the first possible, since the level 8candidate needs 3 RBGs of size 3 and aligning to the last PRB of theRBG#3 is performed. In this case the first aggregation level 8 candidateis mapped on resource block groups 1, 2 and 3 and aligned to the end ofthe resource block group 3. The second candidate of aggregation level 8is distributed. The distribution is performed over aggregation level 2candidates, in particular, the position of distributed 2-PRBs-long level8 candidate portions are mapped on positions of the first, the second,the fifth and the sixth aggregation level 2 candidates.

FIG. 11, part (c) is an example of a search space mapped on resourceblock groups of size 4. Similarly to the previous examples describedwith reference to parts (a) and (b) of FIG. 11, the candidates ofaggregation level 1 are mapped to the first physical resource block ineach of resource block groups. The rest the candidates of aggregationlevel 2 (such as the first candidate 1103) are mapped to the last twophysical resource blocks of each resource block group—they end with thepast PRB of an RBG. The two candidates of aggregation level 4 arelocalised and mapped on the first available resource block group whichmeans resource block group #1 and resource block group #2, respectively.One candidate of aggregation level 8 is localised and mapped onto thefirst available resource block group and aligned to the end of thegroup. Thus it covers resource block groups #1 and #2. The secondcandidate of aggregation level 8 is distributed over aggregation level 2candidates. The distributed 2-PRB large portions of the second candidateare located on two last physical resource blocks of resource blockgroups #1, #2, #5 and #6.

FIG. 11 thus illustrates an embodiment in which the candidates ofaggregation level 1 are located in the first physical resource block ofeach resource block group. Candidates of aggregation levels higher thanaggregation level 1 are aligned with the end of resource block groups.In case the physical resource block group is smaller than the size ofthe localised candidate (cf. for instance parts (a) and (b) of FIG. 11for candidates of aggregation level 4), the candidate will cover morethan one resource block groups. In such a case, it is aligned to the endof the last of the covered adjacent groups. One candidate of theaggregation level 8 is distributed. The portions of the distributedcandidate are also aligned to the end of the resource block groups—theportions are distributed over level 2 candidates.

As already emphasized above in connection with FIG. 10, FIG. 11 onlyprovides an example search space. This example employs 6 candidates ofaggregation level 1, 6 candidates of aggregation level 2, and 2candidates for the respective aggregation levels 4 and 8. This is atypical configuration used, for instance, in LTE. However, the presentinvention is not limited thereto. The number of candidates peraggregation level may be selected according to the requirements of thesystem. Moreover, the basic resource which is allocated to a candidatemay be a control channel element, which may be smaller than the physicalresource block. Then, the aggregation of control channel elements isperformed. The present invention may equally be applied to the controlchannel elements. Moreover, the basic resources (physical resourceblocks or control channel elements) do not necessarily have to begrouped to groups of physical resource blocks or to corresponding groupsof control channel elements. The particular approach of the presentinvention, namely, control space with centralized lower aggregationlevel candidates and distributed higher level candidates, is applicableirrespectively of any such grouping of resources. Similarly, thearrangements for reducing the overlapping between the candidates ofdifferent levels may be applied without any underlying resource blockgrouping.

FIG. 12 shows an alternative embodiment of the present invention for RBGsizes 2 (a), 3 (b) and 4 (c). Accordingly, the candidates of aggregationlevel 1 (such as the first candidate when RBG size 2 is employed 1201)are mapped similarly to the example described with reference to FIG. 11,namely they are mapped on the first physical resource block of eachresource block group. The candidates of aggregation level 2 are mappedstarting from the physical resource block following the physicalresource blocks to which aggregation level 1 candidates were mapped, ifpossible (cf. in case of RBG size 4, the first candidate of level 2,1203). This also corresponds to a shift of one PRB from the start of theRBG. In this example, the candidates of aggregation level 4 are mappedsimilarly as in the previous examples namely by aligning their ends tothe ends of the respective resource block groups (cf. the firstcandidate of level 4, 1202). In FIG. 12, a shift of 1 PRB isillustrated. However, the present invention is not limited thereto andthe candidates of higher aggregation levels may also be shifted by twoor more physical resource blocks from the start of the resource blockgroup or with respect to candidates of another aggregation level.

Such a search space does not differ from the search space described withreference to the example of FIG. 11 for RBG of size 2 and 3. However, adifference can be seen for candidates of aggregation level 2 in resourceblock groups of size 4 (part (c) of the figure). The shift may bedefined with respect to the start of the RBG. Alternatively, the shiftmay be defined with respect to candidates of other aggregation level,for instance, with respect to the closest lower aggregation level.

Another embodiment relates to a search space in accordance with thepresent invention as illustrated in FIG. 13. Accordingly, the twoaggregation level 4 candidates are aligned differently: the firstlevel-4 candidate 1302 is aligned with the first PRB of an RBG and thesecond candidate 1303 is aligned with the last available PRB of an RBG.This configuration enables the balancing of the number of blocked level1 and level 2 candidates. In particular, FIG. 13 shows candidates of theaggregation level 1 (cf. the first candidate of level 1, 1301) mapped tothe first physical resource block of each resource block group andcandidates of aggregation level 2 (cf. the first candidate of level 2,1304) mapped to the last two resource blocks of each resource blockgroups as in previous examples. The different candidates of the samehigher aggregation level may be aligned differently. Apart of thealignments to the start and to the end of the resource block groups,shift defined with respect to the start or end of the RBG may beapplied. Alternatively, a shift with respect to candidates of otheraggregation level may be applied. The shift may differ for candidates ofthe same aggregation level.

The examples described with reference to FIGS. 11, 12 and 13 showed acase in which one higher aggregation level candidate is localised andone candidate of the same level is distributed. The illustratedlocalised and distributed aggregation level was level 8. However, thepresent invention is not limited thereto. More than one distributedcandidate may be supported for a larger aggregation level in order tobetter support multiple relay nodes (receiving nodes) sharing the samesearch space resources. Moreover, any candidates of aggregation level 4may also be distributed. FIGS. 11, 12 and 13 showed mapping of level-1candidates always on the first control channel element/resource block inan RBG. However, the level-1 candidates may also be mapped to the lastcontrol channel element/resource block in an RBG. In such a case, thelevel-2 candidates may start with the first PRB in an RBG.

Regarding the support of multiple relay nodes, according to anotherembodiment of the present invention the positions of distributed higheraggregation level candidates of one receiving node are different from(in particular, orthogonal to) positions of distributed candidates ofthe same level of another receiving node. This may be performed, forinstance, depending on a receiving node ID.

For example, according to 3GPP LTE Release 8 (and later releases), a UEis identified during regular operation by a so-called C-RNTI, which isbasically a value represented by 16 bits. Accordingly, it can be assumedthat a relay node ID, or generally a receiver ID, in the presentinvention can be identified by a similar or same identifier. In onesimple example, one or more of the bits of such an identifier determinewhether and/or how much the position of the higher aggregation levelcandidate(s) are shifted, in terms of physical resource blocks. Forexample, the least significant bit of the relay node ID is used whethersuch candidate(s) are shifted (bit equals 1) or not (bit equals 0) byone physical resource block.

Preferably, the highest aggregation level is distributed in such a way.Advantageously, the positions of localised candidates with a lowernumber of aggregated physical resource blocks are identical for a firstreceiving node and a second receiving node. For instance, theaggregation level 1 and 2 candidates may be mapped onto the samepositions for a plurality of receiving nodes while the 4 and/or 8aggregation level candidates are distributed orthogonally, meaning thattheir positions differ.

In order to reduce the blocking of lower aggregation level by a higheraggregation level and vice versa, in accordance with another embodimentof the present invention, the candidates of higher aggregation levelsare consisting of control channel elements, where the position of atleast a first control channel element is identical to the position of acontrol channel element of a candidate of a first lower aggregationlevel, and where the position of at least a second control channelelement is identical to the position of a control channel element of acandidate of a second lower aggregation level, where the first loweraggregation level size is different from the second lower aggregationlevel size, and both are smaller than the aggregation level size of thehigher aggregation level. For instance, the candidate of higheraggregation level is of AL 8, a first control channel element of thiscandidate is mapped to the position of a candidate of aggregation level1 and the position of a second control channel element of this candidateis mapped to the position of any of control channel elements of acandidate of aggregation level 2. In a further example, the candidate ofhigher aggregation level is of AL 8, a first control channel element ofthis candidate is mapped to the position of one candidate of aggregationlevel 1 and the position of a second and third control channel elementof this candidate is mapped to the positions of one candidate ofaggregation level 2.

In order to reduce the blocking of aggregation levels, in accordancewith another embodiment, the control channel elements for distributedaggregation are mapped to PRBs in RBGs, where at least one PRB is notused for any control channel element belonging to a lower levelaggregation candidate. For example, according to FIG. 13 (c), it can beobserved that within each RBG of size 4, the second PRB is not used bycontrol channel elements for aggregation levels 1 and 2. Consequently,four such PRBs can be used to form a distributed candidate foraggregation level 4, and/or eight. Such PRBs can be used also to form adistributed candidate for aggregation level 8. In this way, the PRBsremaining within RBGs already partially occupied by candidates of loweraggregation level(s) may be further utilized for candidates of higheraggregation level(s).

One issue that may be taken care of when following the above describedrules of mapping is that on one end of the system bandwidth, afractional RBG may exist, in a case where the system bandwidth in termsof resource blocks is not an integer multiple of the defined RBG size.Alternatively, particularly for small RBG sizes, it can occur that theRBG size is smaller than an aggregation level. So this should be takenin account when mapping R-PDCCH candidates onto that RBG. For instance,in FIG. 11 (b), the first level 4 candidates cannot be mapped such thatit end on the third PRB of the first RBG, because then the first controlchannel element of the candidate cannot be mapped to a usable PRB. Thesimplest solution is to disable fractional RBGs from being able to carrycandidates or control channel elements for blind decoding. Anothersolution is for those cases where the above described rules wouldrequire that the start or end of a candidate are outside the availablePRBs, the respective candidate is shifted such that all control channelelements can be mapped on PRBs. So an exception to the rule e.g. in FIG.11 (b) could be that the first candidate for level 4 is not starting atthe third PRB and ends with the sixth PRB, but instead starts at thefirst PRB and ends with the fourth PRB.

In accordance with an advantageous embodiment of the present invention,the receiving node is a relay node and the transmitting node is aneNodeB, the control information is uplink/downlink grant communicatedover R-PDCCH search space configured as described above. The stepsperformed by such a receiving and transmitting node are illustrated inFIG. 14.

In FIG. 14, solid lines represent an embodiment of the present inventionin which a transmitting node maps 1430 control information to be decodedby a receiving node onto a search space. Here, the search space isarranged in accordance with any of the above examples. In particular,the subframes are subdivided into resource units and the controlinformation is blindly decoded from a set of candidates defining thesearch space. A candidate may aggregate a plurality of the resourceunits such as control channel elements or physical resource blocks. Thesearch space in frequency domain includes localised candidates anddistributed candidates. In particular, lower aggregation levelcandidates are localised and higher aggregation level candidates aredistributed in frequency. The transmitter maps the control informationonto the candidates for the particular receiving node and transmits 1440it accordingly. The receiving node blindly decodes 1480 the candidatesof the configured search space and obtains 1480 therefrom the controlinformation, which is further processed in accordance with its purpose.

As described above, the search space may be, in general, configuredfixedly, statically, semi-statically or dynamically. The dashed lines inFIG. 14 illustrates an embodiment of the present invention according towhich the transmitting node first selects 1410 a search spaceconfiguration (i.e. resources available for mapping of the controlchannel carrying the control information and/or the candidates which areto be monitored by the particular receiving node). The selectedconfiguration is then signalled 1420 to the receiving node. Thereceiving node receives 1450 indication of the search spaceconfiguration and sets 1460 the search space to be monitored (blindlydecoded) accordingly. In general, the receiving node may configure thesearch space instead of the transmitting node.

In particular, in view of the LTE embodiments discussed above, thetransmitting node may be the donor eNodeB 510 and receiving node a relaynode 520. The search space configuration in terms of candidates assignedto a particular relay node to monitor may be performed dynamically. Thesearch space configuration in terms of resources available to carryR-PDCCH may be configured semi-statically (for instance, by RRC) orfixed.

FIG. 15 illustrates an example network with two relay nodes served bythe same donor eNodeB. The present invention is particularlyadvantageous for more relay nodes since the search space may efficientlyassign resources to different relay nodes.

However, the present invention and the above embodiments of configuringthe search space are equally applicable to other nodes such as userequipments.

Summarizing, the present invention relates to providing controlinformation within a search space for blind decoding in a multi-carriercommunication system. In particular, the control information is carriedwithin a sub-frame of the communication system, the sub-frame includinga plurality of control channel elements. The control channel elementsmay be aggregated into candidates for blind decoding. The number ofcontrol channel elements in a candidate is called aggregation level. Inaccordance with the present invention, the candidates of loweraggregation levels are localised, meaning that the control channelelements of one candidate are located adjacently to each other in thefrequency domain. Some candidates of the higher aggregation level(s) aredistributed in the frequency.

Further, the various embodiments of the invention may also beimplemented by means of software modules, which are executed by aprocessor or directly in hardware. Also a combination of softwaremodules and a hardware implementation may be possible. The softwaremodules may be stored on any kind of computer-readable storage media,for example RAM, EPROM, EEPROM, flash memory, registers, hard disks,CD-ROM, DVD, etc.

Most of the embodiments have been outlined in relation to a 3GPP-basedarchitecture of a communication system and the terminology used in theprevious sections mainly relates to the 3GPP terminology. However, theterminology and the description of the various embodiments with respectto 3GPP-based architectures are not intended to limit the principles andideas of the inventions to such systems only. Also the detailedexplanations given in the Technical Background section above areintended to better understand the mostly 3GPP specific exemplaryembodiments described herein and should not be understood as limitingthe invention to the described specific implementations of processes andfunctions in the mobile communication network. Nevertheless, theconcepts and sub-frame structures proposed herein may be readily appliedin the architectures described in the Technical Background section.Furthermore, the concept of the invention may be also readily used inthe LTE-A RAN currently discussed by the 3GPP.

1. A transmission method comprising: mapping downlink controlinformation to one of a plurality of candidates included in a searchspace, the search space being configured on a data region of a downlinksubframe, each candidate being a resource for mapping the downlinkcontrol information, and each candidate including a control channelelement (CCE) or a plurality of aggregated CCEs, wherein a firstaggregation level value for localized allocation where CCE(s) includedin a candidate are localized in a frequency domain is smaller than asecond aggregation level value for distributed allocation where CCEsincluded in the candidate are at least partially distributed in thefrequency domain; and transmitting a signal including the downlinkcontrol information mapped to the search space.
 2. The transmissionmethod according to claim 1, wherein the first aggregation level valueis a value indicating a maximum aggregation level available for thelocalized allocation and the second aggregation level value is a valueindicating a maximum aggregation level available for distributedallocation.
 3. The transmission method according to claim 1, wherein thefirst aggregation level value is a value indicating an aggregation levelused for the localized allocation and the second aggregation level valueis a value indicating an aggregation level used for distributedallocation.
 4. The transmission method according to claim 1, wherein thesearch space only includes candidates for the localized allocation oronly includes candidates for the distributed allocation.
 5. Thetransmission method according to claim 1, wherein a single search spaceincludes a candidate for the localized allocation and another candidatefor the distributed allocation.
 6. The transmission method according toclaim 1, wherein the downlink control information includes uplink grantand downlink assignment information.
 7. The transmission methodaccording to claim 1, wherein the first aggregation level value is 1 or2 and the second aggregation level value is 4 or
 8. 8. A transmissionapparatus comprising: a mapper which, in operation, maps downlinkcontrol information to one of a plurality of candidates included in asearch space, the search space being configured on a data region of adownlink subframe, each candidate being a resource for mapping thedownlink control information, and each candidate including a controlchannel element (CCE) or a plurality of aggregated CCEs, wherein a firstaggregation level value for localized allocation where CCE(s) includedin a candidate are localized in a frequency domain is smaller than asecond aggregation level value for distributed allocation where CCEsincluded in the candidate are at least partially distributed in thefrequency domain; and a transmitter which, in operation, transmits asignal including the downlink control information mapped to the searchspace.
 9. The transmission apparatus according to claim 8, wherein thefirst aggregation level value is a value indicating a maximumaggregation level available for the localized allocation and the secondaggregation level value is a value indicating a maximum aggregationlevel available for distributed allocation.
 10. The transmissionapparatus according to claim 8, wherein the first aggregation levelvalue is a value indicating an aggregation level used for the localizedallocation and the second aggregation level value is a value indicatingan aggregation level used for distributed allocation.
 11. Thetransmission apparatus according to claim 8, wherein the search spaceonly includes candidates for the localized allocation or only includescandidates for the distributed allocation.
 12. The transmissionapparatus according to claim 8, wherein a single search space includes acandidate for the localized allocation and another candidate for thedistributed allocation.
 13. The transmission apparatus according toclaim 8, wherein the downlink control information includes uplink grantand downlink assignment information.
 14. The transmission apparatusaccording to claim 8, wherein the first aggregation level value is 1 or2 and the second aggregation level value is 4 or 8.